Many MEMS (Micro-Electro-Mechanical Systems), NEMS (Nano-Electro-Mechanical Systems) and nano devices implement their function by transferring, modulating or transducing mechanical energy. In the process of transfer, modulation and/or transduction of mechanical energy, some energy is dissipated. Such energy dissipation is a consequence of the First and Second Laws of Thermodynamics.
For example, a MEMS cantilever is the simplest embodiment of a MEMS inertial device. The cantilever is set into motion by an external impulse, such as an external inertial force, or through electrostatic coupling. The energy dissipation is quantified by a metric called the attenuation coefficient. When the mechanical response of the cantilever in such devices is a mass-spring-damper system, the energy dissipation term corresponds to the damper in the system.
Another example of a MEMS device that transfers mechanical energy is a MEMS bulk acoustic wave (BAW) resonator. Such resonators are realized in suspended (Film Bulk Acoustic Resonator—FBAR) and solidly mounted (Solidly Mounted Resonator—SMR) configurations.
A metric of energy dissipation in resonating or vibrating devices is the quality factor Q, which is expressed as a ratio of the energy stored to the energy dissipated per cycle:Q=Energy stored/Energy dissipated per cycle   (1).
Both configurations consist of charge collecting electrodes coupled to a piezoelectric transducer. The transducer, usually a piezoelectric layer (but can also be an electrostatic transducer), converts an applied AC electrical stimulus to an AC mechanical strain. The mechanical strain propagates through the device, and is reflected at the boundaries of the device. The “boundary” is a mechanical impedance discontinuity, such as the electrode-air interface in the case of the FBAR, or the electrode-bragg interface in the case of the SMR. This sets up a standing wave within the resonator, which can be represented by a model known as the BVD model. The BVD model is analogous to the spring-mass-damper model used for the inertial cantilever, with an additional parallel capacitance representing the dielectric of the transducer. As before, the damper represents the energy dissipated in the device.
Inertial MEMS structures and BAW resonators have been extensively studied and manufactured. Such structures are formed by patterning the structure in a substrate and releasing the structure therefrom using semiconductor processing techniques.
Such devices have had widespread commercial use. For example, silicon, polysilicon and diamond-based resonators have made it into a multitude of inertial MEMS based sensors manufactured by companies like ST Microelectronics, Bosch, Analog Devices, Invensense, Virtus Advanced Sensors, SiTime, Discera, Sand9 and others. These sensors are found in a wide array of products such as acceleration sensors, gyros, shock sensors, oscillators, chemical and biological sensors, etc.
BAW devices are described in Rosenbaum, T., “RF Bulk Acoustic Wave Filters for Communications” (Artech House, Publ. 1st Ed., May 31, 2009). Quartz-based BAW resonators have been manufactured for many years and more recently piezoelectric materials such as Aluminum Nitride (AlN) and Zinc Oxide (ZnO) have been extensively studied and commercialized in BAW devices. Tunable materials such as Strontium Titanate (STO), Barium Strontium Titanate (BST) and other incipient ferroelectrics (disclosed in commonly owned patent application U.S. Ser. No. 12/391,490), which is incorporated by reference herein, are being examined to complement the functionality of AlN or ZnO in a BAW resonator. AlN-based BAW resonators have been used in RF filter and power amplifier products.
BAW resonators are also being investigated as a frequency reference for oscillators as described in Hashimoto, K. Y., “RF Bulk Acoustic Wave Filters for Communication,” (Artech House Publ. 1st Ed., May 31, 2009). The BAW resonator provides a large change in impedance (magnitude, phase, or both) over a frequency range. A wide range of oscillator architectures can be implemented around such a resonator. However, in all instances, the oscillator performance is enhanced by a high quality (Q) factor resonator.
In MEMS design, one objective is to reduce the dissipated energy as much as possible. This provides a commensurate reduction in operating power, and device lifetime is increased and the desired product performance metrics are more readily achieved. A reduction in dissipated energy can be achieved by improving materials and processes or by improving device geometry. However, although some reduction in dissipation of energy in MEMS devices has been achieved by improving materials and processes, further improvement continues to be pursued.